The present disclosure herein relates to flash memory devices.
Generally, semiconductor memories are a vital microelectronic device in digital logic systems such as computers and microprocessor-based applications ranging from satellites to consumer electronics. Thus, an advance in a manufacturing technology of a semiconductor memory including a process improvement and a technology development obtained through a scaling for a high integration and a high speed is helpful to establish performance standards of different digital logic families.
Semiconductor memory devices are classified into volatile semiconductor memory devices and nonvolatile semiconductor memory devices. In volatile semiconductor memory devices, logic information is stored by setting a logic state of bistable flip-flop in case of a static random access memory (SRAM) and by charging a capacitor in case of a dynamic random access memory (DRAM). In case of volatile semiconductor memory devices, data is stored and read in and from them while a power is applied, but data is lost when the power is interrupted.
Nonvolatile semiconductor memory devices such as MROM, PROM, EPROM, EEPROM or the like can store data even when a power is interrupted. Nonvolatile memory data storage states are permanent or reprogrammable according to a manufacturing technology. Nonvolatile semiconductor memory devices are used for program and microcode storage in a wide range of application such as a computers, avionics, telecommunications, and user electronics. A combination of volatile and nonvolatile memory storage modes in a single chip is also available in devices like a nonvolatile RAM (nvRAM) devices. The devices are used in systems requiring a nonvolatile memory that is fast and reprogrammable. Furthermore, there has been developed a specific memory structure including additional logic circuits to optimize performance for application-oriented work.
In nonvolatile semiconductor memory devices, MROM, PROM and EPROM are not readily writeable or erasable by system itself, so it is not easy for users to update the memory contents. On the contrary, EPPROM can be electrically writeable and erasable, so it is applied into auxiliary memory devices or system programming memories which need continuous update.
As an example of nonvolatile memory devices, a flash memory device is a kind of EEPROM that a plurality of memory areas is erased or programmed by one program operation. In a general EEPROM, only one memory area can be erased or programmed at one time. This means that when systems using a flash memory device read and write with respect to different memory areas at the same time, a flash memory device can operate at a higher and effective speed. All types of flash memory and EEPROM are worn out after the specific number of erasing operations due to an abrasion of an insulating layer surrounding a charge storage means used to store data.